1. Field of the Invention
This invention relates to the preparation of crystalline caustic soda 3.5 hydrate (abbreviated as NaOH.3.5 H.sub.2 O crystals) and more particularly, to the crystallization of NaOH.3.5 H.sub.2 O by cooling in a manner such that the NaOH.3.5 H.sub.2 O crystals do not adhere to the face (wall) of the cooling solution reaction vessel. The present invention is useful for the purification of recovery of caustic soda from crude solutions, such as so-called "cell liquors" produced by the customary electrolytic diaphragm process.
2. Description of the Prior Art
In the past, the industrial manufacture of caustic soda (NaOH) was accomplished by the mercury process, which comprises electrolyzing a sodium chloride (NaCl) aqueous solution, using mercury for the cathode and decomposing the sodium amalgam obtained. An alternate process is the diaphragm process, which eliminates the use of the mercury cathode. The mercury process is better than the diaphragm process, for producing high purity NaOH, but it has the weakpoint that treatment of the electrolysis residue such as mercury mud is required. This causes pollution difficulties.
In the manufacture of caustic soda by electrolysis of NaCl solutions or brine, using diaphragm cells, efficient operation will not permit the conversion of more than about 50 percent of the sodium chloride (NaCl) content of the solution into sodium hydroxide (NaOH). As a consequence, the direct product of the electrolytic operation, i.e., the cathode or so-called cell liquor, is a solution containing about three parts by weight of NaCl to two parts by weight of NaOH; a typical cell liquor made from saturated brine will have approximately the following composition:
1000 parts by weight of NaOH PA1 1500 parts by weight of NaCl PA1 7700 parts by weight of water PA1 1. A 32 to 34% (concentration of liquid phase) aqueous NaOH solution containing NaOH.3.5 H.sub.2 O crystals (see crystals) is prepared. PA1 2. The prepared mixture is cooled to remove the heat of crystallization of NaOH.3.5 H.sub.2 O. The temperature for the cooling of the NaOH solution is from 10.degree. to 0.degree.C. PA1 3. the liquid phase in the slurry is adjusted and maintained usually to 32 to 34% NaOH content, for example, by adding NaOH, H.sub.2 O or NaOH solution, particularly 38 to 40% NaOH solution approximately corresponding to NaOH.3.5 H.sub.2 O, to the above mixture in proportion to the speed of crystallization of the NaOH.3.5 H.sub.2 O crystals. PA1 4. A part of the produced slurry is continuously removed and large cubic NaOH.3.5 H.sub.2 O crystals containing less than 0.15% NaCl are obtained.
The solution may, and usually does, contain impurities other than NaCl, such as iron compounds, sodium sulfate and potassium chloride.
A process for the purification of NaOH, which comprises preparing, at a temperature of about 10.degree.C, a solution containing NaOH in a quantity exceeding that of a saturated solution, but not substantially exceeding 46%, and saturated with NaCl at a temperature in the neighborhood of 30.degree.C, diluting the solution with water to a 38.8% aqueous NaOH solution, which corresponds to a NaOH.3.5 H.sub.2 O solution, and crystallizing the caustic soda from the solution by cooling, is employed for recovering NaOH from such crude solutions thereof.
Crystalline NaOH.3.5 H.sub.2 0 is stable in the temperature range of 5.degree.C to 15.5.degree.C in 32% to 45% NaOH solutions. When the crystals are cooled to less than 5.degree.C in such NaOH solutions, hydrate crystals other than the 3.5 hydrate of NaOH co-exist with the NaOH.3.5 H.sub.2 O crystals and the crystals melt when warmed above 15.5.degree.C.
A caustic soda solution may be strongly supercooled, that is, cooled to a strongly supersaturated condition in the absence of seed crystals without producing crystallization, and in this supersaturated condition the solution is remarkably stable. Caustic soda solutions such as those involved in the process of the invention may be supercooled as much as 20.degree.C, and in this supercooled condition are stable against such known methods of inducing crystallization as agitation, scratching the wall of the receptacle, introducing dust, or the like. However, crystallization can be induced quite readily by the use of seed crystals or by the application of sufficient supercooling.
Moreover, when aqueous NaOH solutions containing NaOH.3.5 H.sub.2 O crystals are cooled, NaOH.3.5 H.sub.2 O crystals will significantly adhere to the face or wall of the receptacle which is being cooled, and thus produce "scaling" of NaOH.3.5 H.sub.2 O crystals.
One examples of the extent of such adherence of the crystals to walls is as follows:
A 1 kg portion of an aqueous 37% NaOH solution was cooled to 11.degree.C in a 2l-glass beaker set in a 10.degree.C cold water bath. To the solution at 10.degree.C was added 1 g of NaOH.3.5 H.sub.2 O crystals (seed crystals) and NaOH.3.5 H.sub.2 O crystallized at once. The temperature of the slurry rose to 14.7.degree.C, because of the heat of crystallization. The slurry was then stirred to increase the yield of produced crystals until the temperature of the slurry was 13.5.degree.C, and the beaker was removed from the cold water bath. The crystalline NaOH.3.5 H.sub.2 O slurry was removed from the beaker, and NaOH.3.5 H.sub.2 O crystals were separated from the slurry and weighed to obtain 272 g (105 g as NaOH). The mother liquor weighed 529 g (190 g as NaOH).
When the slurry was removed NaOH.3.5 H.sub.2 O crystals adhered to the cooling face of the beaker. The thickness of the adhered crystals was 7 to 8 mm. The adhered crystals were melted and weighed 200 g (75 g as NaOH). The amount of NaOH solution obtained by melting the adhered NaOH.3.5 H.sub.2 O crystals was considerable and shows the extent of the adherence of the crystals to the wall.
In the above example, even if the slurry was cooled to adjust for the small difference in temperature between the part in direct contact with the cooling wall and the central part of the slurry, for example, about 1.degree.C of temperature difference between the two parts, many NaOH.3.5 H.sub.2 O crystals still adhered to the cooling wall. Therefore, when NaOH.3.5 H.sub.2 O crystals are produced by merely cooling a slurry containing NaOH.3.5 H.sub.2 O crystals, complicated operations, such as scraping the crystals adhering to the wall of the vessel, have been required in the past for the isolation of the crystals.
Such adherence of the crystals to the wall causes a lowering of the cooling efficiency, difficulty in recovery of crystals and lowering of the purity of the crystals. Therefore such known methods using walls for cooling are inadequate for producing industrially useful high purity NaOH. In order to solve the problem caused by adherence, a process for cooling which comprises the inflow of a gas, such as pressured freon and pressured butane into the slurry and adiabatically expanding the gas, or a process for cooling by vacuum evaporation, which are known special cooling processes, can be employed. However, such processes are not good in view of the high cost of the required closed system apparatus. Moreover, the process using gas is impractical in view of the high cost for recovery of gas, high price of the gas itself, and the loss of the gas. Furthermore, the process for cooling by vacuum evaporation is not advantageous industrially in view of the very large equipment needed for producing vacuums and cooling and because of the high vacuums required for a crystallization temperature of less than 15.degree.C.
A need exists therefore for an efficient, inexpensive method of obtaining high purity NaOH.3.5 H.sub.2 O crystals without adherence to the cooling walls.